BENCHTOP SIMULATION OF DEICER INFILTRATION AND PERCOLATION AS A MODEL FOR CONTAMINANT TRANSPORT

The recent USGS publication SIR 2014-5012 indicates that road deicers contribute 37 and 38 wt. % to annual dissolved solids in New England and Mid-Atlantic states respectively. Deicers are now the #1 contaminant in ground water/surface water systems in the northern US and a study of their transport in the subsurface is critical in developing strategies for contaminant control. We carried a series of benchtop experiments in transparent 12x12x21 cm tanks filled with a single fraction silt or sand and a brine solution introduced at the onset of each experiment into a narrow trench (3x3 mm) at the top of the tank. The small “trench” is prepared alongside the wall and filled with a brine solution dyed with CAS# 3844-45-9 blue dye to enhance brine contrast for observation. Each experiment is captured by high resolution time lapse photography and a video, until a uniform distribution of the brine was reached typically in 2 to 5 days.

The experimental setup creates a gravitational instability caused by a narrow band of denser brine (0.5 to 5 wt. %) overlying lighter fresh water zone beneath it. This 2 layer system leads to Rayleigh-Taylor convection consisting of a general advancing front and a set of 5 to 12 “fingers” and “finger clusters” rapidly moving downward through the underlying fresh water layer. At the end of the experiment an originally fresh water zone is now a concentration gradient zone showing rapid processes of dilution through dispersion along the brine-fresh water interface. The initiation of fingers substantially increases the area of flow-counter flow interactions and the subsequent rapid dilution. Several high resolution photos representing the time sequence of the experiment were analyzed by enhancing the blue channel to quantify the dilution process and map the sequence of brine dilution at different stages of the experiment. The final gradient has concentration approximately 1/10 to 1/20 of the original solution. Field observations of deicer concentration in GW/SW are of the order of hundred to several thousand ppm of dissolved deicer whereas the brine runoffs are in the range of 0.2 to 8 wt. %. The enhanced dispersion along the Rayleigh-Taylor convective interface provide an effective mixing mechanism on a larger scale.